(a) Field of the Invention
The present invention relates to an organic light emitting diode (OLED) display and a method of manufacturing the OLED display, and more particularly, to an OLED display having an improved aperture ratio and a method of manufacturing the OLED display.
(b) Description of the Related Art
An OLED display is a flat display apparatus for driving NxM OLEDs with current or voltage to display an image.
The OLED displays may be classified into active and passive matrix driving types according to driving schemes.
The active matrix driving type of OLED display includes a driving circuit portion and an OLED unit having OLEDs that emit light according to driving signals from the driving circuit portion.
The driving circuit portion includes a plurality of thin film transistors (TFTs) and storage capacitors for driving the OLEDs. Generally, each of the TFTs includes a semiconductor layer, a gate electrode, a source electrode, and a drain electrode.
The semiconductor layer may be made of amorphous silicon (a-Si) or low temperature polysilicon (LTPS). Since mobility of the LTPS is higher than that of the a-Si, the LTPS is more suitable for a high-speed circuit. Therefore, recently, the LTPS has been widely used for the semiconductor layer.
However, a crystallization process of the LTPS is completed at a temperature of 400□ or more.
In a conventional method, in order to prevent damage to an organic layer of the OLED caused by the crystallization process of the LTPS, the driving circuit portion is firstly formed on a substrate, and then the OLED unit is formed on the driving circuit portion.
In addition, the active matrix driving type of OLED displays may be classified into bottom, top, and bottom-top emission types.
In the bottom-emission active matrix driving type of OLED display, light emitting from the organic layer of the OLED passes through a space where the driving circuit portion is not formed so as to display an image. Since the aperture ratio decreases by the space where the driving circuit portion is formed, there is a problem in that the aperture ratio may be lowered to below 40% or less.
On the other hand, in the top-emission active matrix driving type of OLED display, the aperture ratio can be improved by up to 60% to 70%. However, first pixel electrodes of the OLED unit must be made of a reflective conductive material, and second pixel electrode of the OLED unit must be made of a transparent or semi-transparent conductive material.
Dark-point defects may be caused by contamination occurring during a process of generating the first pixel electrodes. In addition, in a case where the second pixel electrodes are formed to be thin so as to improve transmittance, pin holes may occur, and the organic layer may deteriorate.